Many scientific and technological applications will benefit from useful and practical sources of laser radiation in the soft x-ray and/or extreme ultraviolet spectral regions. While lasers operating at longer wavelengths have existed for many years, it was not until recently that soft x-ray lasers were successfully operated. However, those soft x-ray lasers recently demonstrated are very large apparatus requiring as the excitation source very large energy lasers, typically delivering laser pulses of hundreds of Joules to kilojoules at longer wavelengths. The cost, complexity, and size of the equipment required to form such x-ray laser apparatus make them impractical for most technical and scientific applications. Hence, a soft x-ray laser apparatus of approximately the size of current table top ultraviolet excimer lasers would find many commercial as well as scientific applications.
Soft x-ray lasers operating at wavelengths of less than 500 Angstroms have currently been demonstrated by: (a) the use of kilojoule Nd-Glass lasers to create highly ionized plasmas from solid targets such as Selenium and (b) the use of CO.sub.2 lasers with pulse energies of several hundred Joules to excite highly ionized plasmas from solid targets.
It should also be noted that x-ray radiation has been generated by either one of the following types of excitation processes,
(i) Electron impact excitation
In this case energetic electrons from laser created plasmas collide with ions of a certain charge state and excite these ions to create a population inversion between two excited levels of this specie. What follows as a result is common to the operation of most other lasers: the process of stimulated emission causes the radiation corresponding to the wavelength of the radiative transition that links the two levels to amplify in intensity as it travels through the medium.
(ii) Electron ion recombination
A laser created plasma rapidly cools at the end of the excitation pulse by either adiabatic expansion, radiation or electron heat conduction. The rapid decrease in the plasma temperature causes the electrons and ions from the plasma to recombine, creating population inversion between excited levels of the lesser charge ions that result from the recombination process. Again radiation with a photon energy corresponding to the energy difference between the inverted levels is amplified.
A third method in which a laser also is used as an excitation source has also been recently used to generate laser radiation at wavelengths in the vicinity of 1000 Angstroms.
(iii) Excitation of core excited states
In this case short laser pulses in the picosecond to nanosecond range are used to bombard a solid target and produce a powerful incoherent soft x-ray flash lamp. These x-rays are made to interact with a gaseous or metal vapor target producing population inversions by photoionization in core excited states, or creating a plasma in which hot electrons produce inversions by electron impact excitation.
An important aspect of the difficulty in constructing x-ray lasers is the requirement of a powerful energy source capable of depositing very high power densities. In all the methods described above a laser is used as such energy source. Higher power density deposition is achieved by focussing the laser beams with lenses or mirrors and hence they are used to generate plasmas that are used in the ways described above to produce the amplification of soft x-ray or extreme ultraviolet radiation.
The capillary discharge method and apparatus described herein allows a more direct conversion of electrical energy into short wavelength laser radiation, without the need of going through the process of converting electrical energy into long wavelength laser radiation, and then using the short wavelength laser radiation to create a plasma from which the x-ray laser light originates. This represents a very significant improvement with respect to the complexity, size and efficiency of such devices.
The advantage of being able to use a capacitor as an energy source to excite the plasma can be visualized when one realizes that while a kilojoule laser fills an entire room, a capacitor storing the same energy is of the size of a small briefcase.